A typical conventional coating structure of optical fiber is a so-called two-layered coating structure composed of a relatively thin baked layer and an extrusion coating layer as proposed in U.S. Pat. No. 3,980,390 (corresponding to Japanese Patent Publication (Unexamined) No. 125754/1975). The coating materials conventionally used for the baked layer and the extrusion coating layer include silicone resins, epoxy resins, urethane resins, polyamide resins (nylon), polyethylene, and the like. Of these coating materials, thermoplastic resins used for the extrusion coating layer have low melting points and undergo remarkable deterioration due to oxidation at high temperatures, and only fluorine-containing resins that withstands the use at high temperatures of 150.degree. C. or more for long periods of time have been employed as an extrusion coating layer for, in particular, so-called heat resistant coated optical fibers.
According to studies of the present invention, however, it was found that optical fibers in which a fluorine-containing resin is used as an extrusion coating layer have mechanical strength much lower than those coated with polyamides (nylon), polyethylene, etc. as an extrusion coating layer, and hence the optical fibers coated with the fluorine-containing resin cannot be put into practical use. For example, the present inventors have found that optical fiber consisting of a glass fiber having an outer diameter of 125 .mu.m, a baked silicone layer provided thereon in a thickness of about 150 .mu.m and an extrusion coating layer of nylon-12 in a thickness of 250 .mu.m has an average strength of 6.2 Kg, whereas the optical fiber having the same structure as above but having an extrusion coating layer of ETFE (ethylene-tetrafluoroethylene copolymer) in a thickness of 250 .mu.m in place of the nylon-12 layer has an average tensile strength of 3.6 Kg.
The present inventors intensively investigated the above-described decrease in the mechanical strength of optical fibers which is coated with fluorine-containing resins and, as a result, have found that such decrease in the mechanical strength is ascribed to fluorine gas or hydrofluoric acid generated at the time of melt-extrusion of a fluorine-containing resin and completed the present invention.
More specifically, it is believed that fluorine gas of hydrofluoric acid generated during the extrusion coating passes through a first baked layer and reaches surfaces of glass fibers to erode the glass surfaces or destroy chemical bonding between the glass surfaces and the baked layer thereby causing the above-described reduction in mechanical strength. Further, it has been found that the reduction of mechanical strength of the optical fiber having an extrusion coating layer of a fluorine-containing resin is also observed when the optical fiber is irradiated with electron beams. This phenomenon is also considered to be caused by fluorine gas or hydrofluoric acid generated by the electron beam radiation.
According to the studies of the present inventors, in order to prevent or minimize generation of fluorine gas or hydrofluoric acid during melt-extrusion of fluorine-containing resins, it is necessary to lower the extrusion temperature of the fluorine-containing resin to about a melting point of the fluorine-containing resin. At such a low extrusion temperature, the viscosity of the resin during extrusion increases thereby leaving a high residual strain in the coating after extrusion molding. The residual strain then causes shrinkage of coating materials with the passage of time, which results in increase of transmission loss due to so-called microbending.
As a result of further investigation, it has been found that when an adsorbable solid powder such as titanium oxide, calcium carbonate and the like is present in the fluorine-containing resin, fluorine gas or hydrofluoric acid that is generated during melt-extrusion of the resin is adsorbed to the solid powder and is prevented from reaching the glass fiber surface thereby causing no reduction in the mechanical strength of the optical fibers.
The present invention has been completed based on the above-described findings and provides a glass fiber for optical transmission (hereinafter, optical fiber) having an improved mechanical strength and comprising a glass fiber having provided thereon a first coating layer of a hardened resin and a second fluorine-containing resin layer formed by extrusion molding, at least one of the first and second layers containing an adsorbable solid powder in an amount of 0.05 to 20% by weight based on the weight of the first or second layer.